1. Field of the Invention
The present invention relates to a fiber laser, and more particularly, to a fiber laser system including a non-oxide fiber as a gain medium and fiber having dysprosium for 3-μm lasing.
2. Description of the Related Art
In a fiber laser, pump light and laser light propagate through a fiber. Therefore, pump light (i.e., light generated by a pump light source) can be efficiently converted, and a resonator can be simply constructed since it is not necessary to align optical components. Furthermore, since the alignment of the resonator is not readily distorted, the output light of the fiber laser is stable, and the mode characteristics of the output light of the fiber laser are good. In addition, since the output end of fiber can be freely moved, the fiber laser can be conveniently used. Examples of the fiber laser include an ytterbium (Yb) fiber laser emitting light having a wavelength of about 1 μm, an erbium (Er) fiber laser emitting light having a wavelength of about 1.5 μm, and a thulium (Tm) fiber laser emitting light having a wavelength of about 2 μm. Such fiber lasers have high output power or are wavelength tunable. Thus, the fiber lasers are widely commercialized and used in, for example, industrial, medical, military, and scientific fields. Silica-based fiber is mainly used for the fiber lasers owing to its low light loss and heat-resisting characteristics. Furthermore, the silica-base fiber is suitable for high output power fiber lasers since the technology for manufacturing optical devices of a silica-based fiber laser is well-developed and silica-based fiber cables can be easily connected by fusion splicer.
However, silica-based fiber lasers have increased light loss when emitting light having a wavelength of about 2 μm or higher and are difficult to operate in a mid-infrared wavelength band since the possibility of non-radiative transition is high due to large phonon energy.
The mid-infrared wavelength band ranges from about 2 μm to about 20 μm and is considered suitable for medical, military, and environmental applications. Particularly, absorption of water molecules is maximal at a wavelength of about 3 μm. Therefore, a biological body having a high water ratio can be easily cut using laser light having a wavelength of about 3 μm even when lasing power is low. A laser capable of generating light of about 3 μm can be usefully used for medical instruments such as a laser scalpel. Currently, lasers such as 2.7-μm Er:YSGG lasers and 2.9-μm Er:YAG lasers are commercialized and have limited applications in medical fields. Due to the limitations of bulk solid-state lasers, research has been ceaselessly conducted on fiber lasers capable of lasing at about 3-μm. For example, laboratory-level research articles have been published on Er (erbium):ZBLAN (Zirconium Barium Lanthanum Aluminum Sodium Fluoride) or Ho (holmium):ZBLAN fiber lasers that are made by doping fluoride-based ZBLAN fiber with a rare-earth element such as erbium (Er) or holmium (Ho). However, such fiber lasers generate laser light having a wavelength in the range from about 2.7 μm to about 2.9 μm, which does not correspond to the wavelength of about 3 μm where absorption of water molecules is maximal.
Since dysprosium has an energy level suitable for lasing at a wavelength of about 3 μm for maximal absorption of water molecules, the dysprosium is considered one of the most applicable rare-earth materials for medical fiber lasers. Accordingly, some articles have disclosed Dy:ZBLAN lasers that are formed using ZBLAN fiber doped with dysprosium and are capable of emitting laser light having a center lasing wavelength of about 3 μm by light pumping. In the article “Applied Physics Letters, pp. 1316-1318 (2003)” by a research team of Sydney University, Australia, a fiber laser capable of lasing at 3 μm is realized by absorption of 1.1 μm pump light to 6H7/2 and 6F9/2 energy levels. However, the slope efficiency of the disclosed fiber laser is low at about 4.5% since electrons at excited energy level absorb the pump light (excited state absorption, ESA). Furthermore, the maximum output power of the disclosed fiber laser is low at about 0.3 W. In another article “Optics Express, pp. 678-685 (2006)” by a research team of Manchester University, England, a fiber laser capable of lasing at 3 μm is realized by absorption of 1.3 μm pump light to 6H9/2 and 6F11/2 energy levels. The latter fiber laser shows less ESA compared with the former fiber laser using 1.1 μm pump light, and thus the slope efficiency of the latter fiber laser is relatively high at about 45%. However, the maximum output power of the latter fiber laser is still low at about 0.2 W. Therefore, there is a need for a high-efficiency, high-power fiber laser including fiber doped with dysprosium for 3 μm lasing.